1. Field of the Invention
The present invention relates to a single crystal laser material. More specifically, the present invention relates to a single crystal active media for solid-state lasers operating in the middle-infrared (Mid-IR) 4-5 μm range of optical spectrum.
2. Description of the Related Art
Solid state lasers operating in the middle-infrared (Mid-IR) 4-5 μm range of optical spectrum are preferable for practical reasons. Laser radiation of 4-5 μm spectral range is essential for many scientific, technological and defense related military applications. This radiation corresponds to so-called “window of atmospheric transparency” of the Earth, which in turn leads to small losses when the radiation propagates in the atmosphere. Lasers and lasing devices operating in this spectral range are advantageous for LIDAR (broadly a laser radar or light direction and ranging and sensing) applications, free space communication, environmental monitoring, and in many other industrial uses.
The word “laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. Lasers are finding an ever increasing number of defense applications, for example in the involving target acquisition, fire control, and training. These lasers are termed rangefinders, target designators, and direct-fire simulators. Lasers are also being used in communications, laser radars (LIDAR) (as noted above), landing systems, laser pointers, guidance systems, scanners, metal working, photography, holography, medicine and in many other ways.
As noted earlier, LIDAR is an acronym for light direction and ranging, and is often a laser remote sensing technique used in science, industry, defense, and homeland security. As a remote sensing technique, it is broadly the optical equivalent of the microwave radar, and so is often referred to as laser radar. In an Atmospheric-Optics Laboratory, lidars are used for atmospheric research, and to obtain measurements of aerosol particulates, clouds, temperatures and water vapor. For example, Lidar measurements are useful in the study of transboundary pollution transport and Arctic climate change. For an example of a practical lidar system (including the certain technical details and measurements), see the Dalhousie Raman Lidar (http://aolab.phys.dal.ca/pages/DalhousieRamanLidar) or the MIT Firepond Lidar (http://www.haystack.mit.edu/).
In common practice, a lidar transmitter is conventionally a powerful green (Nd:YAG)-type laser, and transmits a short and intense pulse of light. The pulse is expanded to minimize its divergence, and is directed by a mirror into the atmosphere. As the pulse travels into the atmosphere it is scattered by atmospheric constituents (mostly nitrogen) and aerosol particulates. Light that is backscattered in the field-of-view of a co-aligned telescope is collected and channeled toward detectors by a fiber or other optical devices. The amount of light received is measured as a function of time (or distance) using sensitive photo-detectors, and the signals are digitized. Coordination of the experiment is performed by a timing unit. When each laser pulse exits the atmosphere, another pulse is transmitted and the process is repeated.
As this practical example details, lidars are valuable instruments for atmospheric research because they provide an active remote sensing technique that can probe atmospheric regions inaccessible to other instruments. Those of skill in the art will recognize that there are many applications of lidar/rangefinder/sensing technology that are used in science, industry, and the military.
The related art involves the achievement of outstanding results in the development of effective lasers operating in ultraviolet (UV), visible (Vis), and near infrared (NIR) (up to 3 μm) range of optical spectrum. In contrast to these UV-Vis-NIR results, lasers of the mid-IR spectral range are not so mastered and functionally and practically are not available.
Very well developed oxide laser crystals are not appropriate as mid-IR laser hosts because of their extended phonon spectrum and correspondingly high rate of multi-phonon nonradiative relaxation of excitation. This issue is discussed more fully by T. T. Basiev, Yu. V. Orlovskii, B. I. Galagan, M. E. Doroshenko, I. N. Vorob'ev, L. N. Dmitruk, A. G. Papashvili, V. N. Skvortsov, V. A. Konyushkin, K. K. Pukhov, G. A. Ermakov, V. V. Osiko, A. M. Prokhorov, Evaluation of rare-earth doped crystals and glasses for 4-5 μm lasing, in Laser Physics, 12, No 5 (2002) p. 859-877 and S. A. Kutovoi, “Growth and laser properties of rare-earth doped lanthanum-scandium borate single crystals” in Physical-Chemical Aspects Of Technology Of Complex Oxides For Solid State Lasers, Moscow: HayKa, 2002. pp. 128-167. (General Physics Institute Proceedings, OΦAH; v. 58), the contents of which are fully incorporated herein by references.
Similarly, halide crystal hosts (being materials with relatively short phonon spectrum) were not very successful for lasing in the mid-IR range probably due to a relatively low oscillation strengths or cross-sections of the mid-IR emission. These issues are discussed by Yu. V. Orlovskii, T. T. Basiev, K. K. Pukhov, N. A. Glushkov, O. K. Alimov, S. B. Mirov, in Multiphonon relaxation of mid IR transitions of rare-earth ions in fluorite type crystals, Advanced Solid State Photonics Conference, Technical digest, Feb. 1-4, 2004, Santa Fe, USA, WB9. OSA TOPS, the contents of which are fully incorporated herein by reference.
The major difficulty in development of solid state mid-IR laser relates to the fact that relatively small amounts of crystalline hosts possess the necessary combination of properties.
In addition to standard requirements that are imposed upon the hosts for UV-Vis-NIR lasers, crystalline hosts for Mid-IR lasing should possess a very low energy of optical phonon cutoff thus minimizing the nonradiative decay rate and improving the quantum yield.
Those of skill in the solid state arts should, for efficiency reasons, like to have the upper laser level of RE ions in these crystals to be purely radiative with rather large oscillator strength of the laser transition for achieving necessary gain in the 4-6 μm spectral range.
The closest analog of the proposed gain medium is single crystal laser material according to the chemical formula CaGa2S4:Dy3+, as noted by M. C. Nostrand, R. H. Page, S. A. Payne, W. F. Krupke, and P. G. Shunemann in Optics Letters, vol. 24, N 17, pp. 1215-1217, 1999, the contents of which are fully incorporated herein by reference.
This is the first laser medium where laser oscillations of dysprosium ions have been achieved on transition 6H11/2-6H13/2 at 4.12 μm with a maximum output energy of 0.12 mJ. The drawback is that the wavelength of this laser corresponds to strong absorption of CO2 molecules and as a consequence experiences strong absorption while propagating in the Earth atmosphere.
What is not appreciated by the prior art is the need for a single crystal laser material with a high cross-section of optical transition, a wavelength shifted to avoid CO2, and a method for fabrication of the same.
Accordingly, there is a need for an improved single crystal laser material that responds to the needs noted above.